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phytes and parasites should be replaced by one which takes into account other peculiarities in the inode of nutrition of bacteria. The nitrifying, nitrogen-fixing, sulphur- and iron-bacteria he regards as monotrophic, i.e., as able to carry on one particular series of fermentations or decompositions only, and since they require no organic food materials, or at least are able to work up nitrogen or carbon from inorganic sources, he regards them as primitive forms in this respect, and terms them Prototrophic. They may be looked upon as the nearest existing representatives of the primary forms of life which first obtained the power of working up non-living into living materials, and as playing a correspondingly important role in the evolution of life on our globe. The vast majority of bacteria, on the other hand, which are ordinarily termed saprophytes, are saprogenic, i.e., bring organic material to the putrefactive state—or saprophilous, i.e., live best in such putrefying materials—or become zymogenic, i.e., their metabolic products may induce blood-poisoning or other toxic effects (facultative parasites) though they are not true parasites. These forms are termed by Fischer Metatrophic, because they require various kinds of organic materials obtained from the dead remains of other organisms or from the surfaces of their bodies, and can utilize and decompose them in various ways {Polytrophic), or, if monotrophic, are at least unable to work them up. The true parasites—obligate parasites of de Bary-—are placed by Fischer in a third biological group, Paratrophic bacteria, to mark the importance of their mode of life in the interior of living organisms where they live and multiply in the blood, juices, or tissues. The growth of an ordinary bacterium consists in uniform elongation of the rodlet until its length is doubled, Growth *°^owed by division by a median septum, then by the simultaneous doubling in length of each daughter cell, again followed by median division, and so on (Figs. 4, 6). If the cells remain connected the resulting filament repeats these processes of elongation and subsequent division uniformly so long as the conditions are maintained, and very accurate measurements have been obtained on such a form, e.g., B. ramosus. If a rodlet in a hanging drop of nutrient gelatine is fixed under the microscope and kept at constant temperature, a curve of growth can be obtained recording the behaviour during many hours or days. The measured lengths are marked off on ordinates erected on an abscissa, along which the times are noted. The curve obtained on joining the former points then brings out a number of facts, foremost among which are (1) that as long as the conditions remain constant the doubling periods—i.e., the times taken by any portion of the filament to double its length—are constant, because each cell is equally active along the whole length; (2) there are optimum, minimum, and maximum temperatures, other conditions remaining constant, at which growth begins, runs at its best, and is soon exhausted, respectively ; (3) that the most rapid cell-division and maximum growth do not necessarily accord with the best conditions for the life of the organism ; and (4) that any sudden alteration of temperature brings about a check, though a slow rise may accelerate growth (Fig. 5). It was also shown that exposure to light, dilution or exhaustion of the food-media, the presence of traces of poisons or metabolic products check growth, or even bring it to a standstill; and the death or injury of any single cell in the filamentous series shows its effect on the curve by lengthening the doubling period, because its potential progeny have been put out of play. Hardy has shown that such a destruction of part of the filament may be effected by the attacks of another organism. Among the most important advances in our know-


ledge of the bacteria are those having reference to the circulation of nitrogen on the globe. When we reflect that some hundreds of thousands of tons bacteria of urea are daily deposited, which ordinary plants are unable to assimilate until considerable changes have been undergone, the question is of importance, What happens in the meantime ? In effect the urea first becomes carbonate of ammonia by a simple hydrolysis brought about by bacteria, more and more definitely known since Pasteur, van Tieghem, and Cohn first described them. Lea and Miquel further proved that the hydrolysis is due to an enzyme—urase—separable with difficulty from the bacteria concerned. Many forms in

rivers, soil, manure heaps, Ac., are capable of bringing about this change to ammonium carbonate, and much of the loss of volatile ammonia on farms is preventible if the facts are apprehended. The excreta of urea alone thus afford to the soil enormous stores of nitrogen combined in a form which can be rendered available by bacteria, and there are in addition the supplies brought down in rain from the atmosphere, and those due to other living debris. The researches of the decade 1890-1900 demonstrated that a still more inexhaustible supply of nitrogen is made available by the nitrogen - fixing bacteria of the soil. There are in all cultivated soils forms of bacteria which are capable of forcing the inert free nitrogen to combine with other elements into compounds assimilable by plants. This was long asserted as probable before Winogradsky showed that the conclusions of Berthelot, Laurent, and others are right, and that Clostridium pasteurianum, for